1. Field of the Invention
The present invention relates to an improvement in a process for producing superconducting films. More particularly, it relates to an improved process for producing thallium type superconducting thin films of thallium containing-compound oxides such as Tl-Ba-Ca-Cu type oxide having higher critical current density (Jc) as well as a high critical temperature (Tc).
2. Description of the Related Art
The superconductivity is a phenomenon which is explained to be a kind of phase change of electrons under which the electric resistance becomes zero and the perfect diamagnetism is observed. Under the superconducting condition, electric current of high density flows continuously without any loss of power, so that the power loss of about 7% which is lost in the electric power transmission today can be saved greatly when the technology of superconductivity can be applied to the electric power transmission. Development of superconductor is also demanded in the field of measurement in order to detect very weak magnetism by SQUID, in the field of medical treatment by .pi.-neutrons and in the field of high-energy physical experiments. Superconductor is also requested in the field of electromagnets for generating a strong magnetic field in order to develop the technologies of fusion power generation, MHD power generation, magnetic levitation trains and magnetically propelling ships. The critical temperature "Tc" of superconductors, however, could no exceed 23.2 K of Nb.sub.3 Ge which was the highest Tc for the past ten years.
The possibility of an existence of new types of superconducting material having much higher Tc was revealed by Bednorz and Muller, who discovered a new oxide type superconductors in 1986 (Z. Phys. B64, 1986 p. 189).
The new type compound oxide superconductor discovered by Bednorz and Muller is represented by [La,Sr].sub.2 CuO.sub.4 which is called the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type command oxides show such higher Tc as 30 K which is extremely higher than known superconducting materials. After that, a variety of compound oxides which show much higher critical temperatures were reported and hence the possibility of an actual utilization of the high Tc superconductors have burst onto the scene.
C. W. Chu et al. reported another superconducting material of so-called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature of about 90 K (Physical Review Letters, Vol. 58, No. 9, p. 908). Maeda et al reported the other type new superconducting compound oxide of Bi-Sr-Ca-Cu-O system (Japanese Journal of Applied Physics, Vol. 27, No. 2, pp. 1209 to 1210).
Thallium type compound oxides are also high Tc superconductors of more than 100 K. The present inventors disclosed several kinds of thallium type compound oxides superconductors in a U.S. patent application Ser. No. 223,634 filed on Jul. 25, 1988 and Hermann et al. reported Tl-Ba-Ca-Cu-O system in Appl. Phys. Lett. 52 (20) p. 1738). Thallium type compound oxides are chemically more stable than the abovementioned YBCO type compound oxide and have such a very important merit that high Tc superconductors of higher than 100 K can be realized without using rare earth elements as a material so that the production cost can be reduced.
The above-mentioned new types oxide superconducting materials can be prepared in a form of a thin film on a substrate by physical vapour deposition (PVD) technique or chemical vapor deposition (CVD) technique.
In the case of production of thallium type oxide superconductors, however, there is a special problem, because thallium (Tl) is a very volatile element and toxic for humans. In fact, thallium type superconducting thin films prepared by the conventional physical vapour technique show relatively lower Tc (critical temperature) and Jc (critical current density) than a balk or a block of this compound oxide which is prepared by sintering technique. It is thought that this difference may be caused by shortage of oxygen in the crystal of which the superconducting thin film is made.
Heretofore, it is a usual practice to anneal a deposited thin film at 600.degree. to 900.degree. C. in the presence of oxygen gas in order to improve the superconducting properties of oxide type superconducting thin films deposited. This technique, however, is not effective in the case of thallium type superconducting thin films because thallium has a high vapour pressure and hence a majority of thallium atoms in the thin film escape from the thin film, so that the resulting thin film does not show desired high Tc and Jc.
Therefore, an object of the present invention is to overcome the problems of the prior art and to provide an improved process for producing superconducting thin films of thallium (Tl) type compound oxides.